Watch cases of ordinary or smartwatches can be made of plastics. A watch case can be installed to a watch lug using a standard strap. When the materials of the watch lug and the watch case are plastic and they are integrally manufactured and processed, due to limitations of plastic molding injection technology, an exterior surface of the plastic watch lug can have several parting lines that can be complicated to be removed and affect the appearance. The parting lines can be treated by grinding and spray painting, but it is uncomfortable for an individual to wear the spray-painted watch. Therefore, a watch lug structure that does not need spray painting and has appealing exterior is sought for.